13 December 2010

The lonely places: Where could life exist, but doesn’t?

Our planet is covered with life. Birds fly over Mount Everest; ecosystems thrive at hot vents at the bottom of the ocean. And the more we have looked, the more and more weird places we find organisms living in places we thought was completely uninhabitable.

Given all the interest in the idea that life could exist without phosphorus, this new article by Cockell is extremely timely. Cockell points out that if we want to understand life, we have to pay attention not just to where life is present, but to where it is absent.

Think about it: where is there no life on this planet? Clearly, there are some places that life cannot be sustained at all. The inside of an active volcano comes to mind. Indeed, volcanoes might a source of what Cockell calls “vacant habitats,” places where life could live, but currently doesn’t. But “sterilization” events are perhaps not very informative, as most places become recolonized.

Cockell is after bigger game, namely places that are routinely uninhabited by life. He doesn’t provide any clear examples of such places on Earth outside of disaster areas where life is temporarily eradicated.

Indeed, we may have a problem even identifying a vacant habitat on Earth. Given that life is so ubiquitous, are “extreme” habitats without life because life cannot be supported at all? Or is it within the realm of life’s capabilities, but there isn’t life in that niche?

Then, Cockell gets more speculative, wondering if other planets might have vacancies for life. He logically divides the possibilities into planets where life never originated (so the whole planet is vacant), and those for which life did originate, but was either localized or wiped out.

To my surprise, although Mars comes up tangentially, Cockell doesn’t make much of it. It seems to me we know enough about the Martian landscape to make a reasonable guess that at least some of it could support terrestrial life. Science fiction fans will no doubt know of many books describing the terraforming of Mars. (The article does have a box dealing with the ethics of introducing life into vacant habitats.)

The perpetual speculation about whether there is microbial life on Mars suggests that Mars is a vacant habitat, in at least one sense. I wrote this a while ago (and it remains one of the favourite things I’ve ever written), shortly after the “Martian meteorite” (McKay et al. 1996) had been announced as a possible indication of past life on Mars:

There is a more depressing side to the announcement of possible past Martian life, however. Mars may be an entire biosphere that has gone extinct. We find living organisms living and often thriving in our planet’s most hostile locations, so terrestrial life appears marvelously tenacious and resilient. There is no evidence of life on Mars now, suggesting that if life originated on the red planet, it never managed to get a toehold: No macroscopic organisms, no increasing complexity, no smart Martians carved out by the forces of natural selection.

While as a biologist, even microbes would be a spectacular finding, the question of whether habitats are vacant for complex, multicellular life is almost as interesting. And if Mars is ever found to support microbial life, why doesn’t it support macroscopic life?